The human B cell alloantigens which appear related to the Ia antigen system of mice will be studied with emphasis on delineating their genetic fine structure, the molecular basis of the alloantigen specificities, and on the relationship of the Ia system to the inherited susceptibility to certain diseases. Hybridoma reagents will be developed using homozygous donors and their reactivities will be compared to those obtained with allo reagents. The expression of Ia antigens on cells other than those of the immune system will continue to be explored as a reflection of stage of differentiation.